The present invention relates to a method for integrating devices (elements) such as transistors by using thin film semiconductor, and more particularly to a method for producing plural thin film devices by using a linear laser beam with no dispersion of characteristics thereof, and also to a thin film device produced by the technique.
Recently, various studies have been increasingly made on reduction in a temperature of a producing process of semiconductor devices because it is required that semiconductor devices must be formed on an insulating substrate such as glass which is low in cost and has high processability. The reduction of the producing process temperature of semiconductor devices is also required to promote microstructure design of devices and multilayer structure of devices.
In a semiconductor producing process, it is often required to perform crystallization of amorphous components contained in a semiconductor material or an amorphous semiconductor material, restoration of crystallinity of a semiconductor material which is originally crystalline, but reduced in crystallinity due to irradiation of ions, or further improvement of crystallinity of a semiconductor material having crystalline. For these requirements, thermal annealing is utilized. When silicon is used as a semiconductor material, the crystallization of amorphous material (components), the restoration of crystallinity, the improvement of crystallinity and the like are performed by annealing at 600xc2x0 C. to 1100xc2x0 C. for 0.1 to 48 hours or more.
In the thermal annealing, the processing time may be set to a shorter value as the process temperature increases, however, no effect can be obtained at 500xc2x0 or less. Thus, for the reduction of the process temperature, it is required to replace the process based on the thermal annealing by another method. In particular, when a glass substrate is used, since the heat resistance temperature of the glass substrate is about 600xc2x0 C., the other method is required to be comparable with the conventional thermal annealing at the process temperature of 600xc2x0 C. or less.
As a method of satisfying the requirement is known a technique of performing various annealing treatments by irradiating a laser light to a semiconductor material. Much attention is paid to the laser light irradiation technique as a ultimate low temperature process. This is because the laser light can be irradiated into only a desired limited portion with high energy which is comparable with the energy of the thermal annealing, and also it is not needed to expose the overall substrate to a high temperature.
Two methods have been mainly proposed for the laser light irradiation. In a first method, a continuous oscillation laser such as an argon ion laser is used to irradiate a spotshaped beam onto a semiconductor material. The semiconductor material is melted and then gradually solidified due to the difference of an energy distribution within a beam and the movement of the beam, to crystallize the semiconductor material. In a second method, a large energy laser pulse is irradiated onto a semiconductor material using a pulse oscillation laser such as an excimer laser, and then the semiconductor material is instantaneously melted and solidified to progress crystal growth of the semiconductor material.
The first method has a problem that the processing needs a long tune. This is because the maximum energy of the continuous oscillation laser is limited and thus the size of the beam spot is set in mm-square order at maximum. The second method has extremely large maximum laser energy, and mass production can be more improved by using a spot beam of several centimeters square or more.
However, when a substrate having a large area is processed with a square or rectangular beam usually used, the beam must be moved in right and left directions and in up and down directions. Thus, it needs further improvement in mass production.
The great improvement can be performed by a method of deforming the beam in a line shape, setting the width of the beam to exceed the length of the substrate to be processed and scanning the beam relatively to the substrate. The term xe2x80x9cscanningxe2x80x9d means that the linear laser is superposedly irradiated while displaced little by little.
However, in the technique of superposedly irradiating the linear pulse laser while displaced little by little, stripes are necessarily formed on the surface of the semiconductor material to which the laser beam is irradiated. These stripes have a large effect on the characteristics of devices which are formed or will be formed on the semiconductor material. Particularly, this effect is critical when plural elements must be formed on a substrate so that the characteristic of each device is uniform. In such a case, the characteristic of each stripe is uniform, there occurs dispersion in characteristic between stripes.
There is a problem with respect to uniformity of the irradiation effect in the annealing using the line-shaped laser light. High uniformity means that the same device characteristics can be obtained over a substrate when devices are formed at any portions on the substrate. Improvement of uniformity means that crystallinity of a semiconductor material is made uniform. The following manner is used to improve the uniformity.
It has been known that, to moderate nonuniformity of the laser irradiation effect and improve its uniformity, it is better to preliminarily irradiate a weaker pulse laser light (hereinafter referred to as preliminary irradiation) before irradiation of an intense pulse laser light (hereinafter referred to as main irradiation). This effect is very high, and it can reduce the dispersion of the characteristics and thus remarkably improve the characteristics of a semiconductor device circuit.
The reason why the preliminary irradiation is effective to obtain the uniformity of a film resides in that a film of a semiconductor material containing an amorphous portion has such a property that the absorbtance of the semiconductor material to laser energy is very different from that of a polycrystalline film or a single crystalline film. That is, a two stage irradiation acts as follows: the amorphous portion remaining in the film is crystallized by a first irradiation process, and then the whole crystallization is promoted by a second irradiation process. By promoting the crystallization moderately, the nonuniformity of stripes occurring on the semiconductor material due to the linear laser irradiation can be suppressed to some degree. Thus, the uniformity of the irradiation effect of the laser light can be remarkably improved, and the stripes are made visually relatively inconspicuous.
However, when a large number of (in several thousands or several ten thousands order) semiconductor devices such as thin film transistors (TFTs) are formed on a glass substrate, for example, in an active matrix type liquid crystal display, no satisfaction can be obtained in the uniformity of the effect even when the laser irradiation method based on the two stage irradiation is used.
As described above, the annealing using an excimer laser light which is processed into a linear beam is excellent from the viewpoint that it can be matched with a large area device design, however, it has a disadvantage in the uniformity of the effect.
An object of the present invention is to provide a technique of extremely suppressing dispersion of characteristics of each semiconductor device when a large number of semiconductor devices are formed by annealing with irradiation of a laser light processed into a linear beam.
Use of a linear laser beam necessarily causes striped nonuniformity. Thus, according to the present invention, the above problem can be overcome by converting a technical concept of improving the uniformity of a semiconductor material to a technical concept of conforming (matching) devices to be formed on or formed on the semiconductor material to nonuniformity due to laser irradiation.
FIG. 1 quantitatively shows a measurement result of nonuniformity which occurs on the surface of a semiconductor material due to the two stage irradiation of the laser light. A KrF excimer laser (wavelength of 248 nm, pulse width of 30 ns) processed into a linear beam of 1 mm width and 12 5 mm length is irradiated onto an amorphous silicon film of 500 xc3x85 thickness formed on a glass substrate while scanned in a direction perpendicular to the line of the beam, and then refractive index of the silicon film is measured.
In FIG. 1, a scan direction represents a refractive index distribution in the scan direction of the linear laser beam, that is, in the direction perpendicular to the line. A beam lateral direction represents a refractive index distribution in the line direction (longitudinal direction) of the linear laser beam. The amorphous silicon film is crystallized by irradiation of the laser light, and variation of crystallinity thereof can be measured on the basis of variation of refractive index thereof. The refractive index of the silicon film can be measured by an ellipsometer if the thickness of the thin film is known. The data in FIG. 1 are obtained when the two stage irradiation is performed.
From FIG. 1, uniformity of a refractive index is more excellent on a line parallel to the linear laser indicated by square marks than that on a line which is substantially vertical to the linear laser beam (the scanning direction of the laser light indicated by circular marks). The refractive index is closely related to crystallinity of the film, and no dispersion of refractive index means no dispersion of crystallinity. Thus, it is concluded that uniformity of crystallization on the line which is parallel to the linear laser is still better than that on the linear which is perpendicular to the linear laser. Also, the anneal effect due to the linear laser light is excellent in the line direction because there is no dispersion, however, there is large dispersion in the scan direction.
The dispersion in the line direction of the linear laser is about 0.6%. However, the dispersion in the scan direction is 1.3% which is above twice larger than 0.6%. Thus, when the annealing is performed with the linear laser beam while scanning the linear laser beam in the direction perpendicular to the line, the anneal effect in the line direction is above twice higher than that in the scan direction perpendicular to the line.
The same is expected to be satisfied for not only the silicon semiconductor thin films, but also for other thin film semiconductors. The effect due to the laser light irradiation in FIG. 1 is applied not only to crystallization of the amorphous silicon films, but also to crystallization of semiconductor thin films, increment and improvement of crystallinity, activation of doped impurities and the like.
The invention is more effective particularly when there is a double or larger difference between the anneal effect in the line direction and the anneal effect in the scan direction in various annealing treatments using linear laser beams.
When plural devices formed on semiconductor material are produced, a circuit design is set so that devices required to have the same characteristics are aligned in a line as much as possible, and a linear laser light is irradiated onto an device area (or an area which will become an device area) in which the devices are aligned to perform various annealing treatments. Thus, the anneal effect of the laser light can be made uniform over each device area in which devices are aligned, and the characteristics of the plural aligned devices have no dispersion.
When a crystal silicon film is formed with a linear laser beam having an anneal effect in FIG. 1 and then a thin film transistor (TFT) is formed using the crystal silicon film, the line connecting the source and drain of the TFT is set to coincide or substantially coincide with the line direction (longitudinal direction) of the linear laser light, whereby the crystallinity in a carrier moving direction can be made uniform. In this case, since carriers move in an area having uniform crystallinity, there is no obstacle (electrical obstacle) to the movement of the carriers and thus the characteristics can be improved.
In the invention, a semiconductor producing method includes the steps of, performing an annealing by irradiating a linear laser light onto a thin film semiconductor, and forming a plurality of semiconductor devices along the longitudinal direction of an area to which the linear laser light is irradiated.
The above steps are used, when TFTs are formed on a substrate having an insulating surface such as a glass substrate. The linear laser light may be formed by shaping an excimer laser light in a linear form through an optical system as indicated in an embodiment described later. The longitudinal direction along which the laser beam is irradiated means the line direction of the area on which the laser beam is irradiated in the linear form.
In the invention, the semiconductor producing method includes a step of irradiating a linear laser light to a thin film semiconductor, wherein the linear laser light is irradiated onto an area in which a plurality of semiconductor devices are aligned at least in a line so that the line direction of the linear laser light coincides with the alignment direction of the devices.
In the invention, the semiconductor producing method includes a step of irradiating a linear laser light to a thin film semiconductor, wherein the linear laser light having a linear pattern is irradiated along a direction which coincide with a direction connecting an area where the source region of a TFT is formed and an area where the drain region of the TFT is formed.
The TFT may be any one of a stagger type, an inverse-stagger type, a planar type and an inverse-planar type. It is particularly effective when a planar type TFT which each of source, channel and drain regions are formed in one active layer is used. The laser light is irradiated for crystallization, promotion of crystallization, improvement of crystallization, activation of impurities, and various annealing treatments.
In the invention, a semiconductor producing method includes the steps of, irradiating a linear laser light onto a thin film semiconductor, and forming a TFT having a source region and a drain region along the line direction of the linear laser light.
In the invention, a semiconductor producing method includes the steps of, irradiating a linear laser light onto a thin film semiconductor, and producing a semiconductor device in which carriers move along the line direction of the linear laser light.
In the invention, a semiconductor producing method includes the steps of, implanting an impurity ion for providing one conduction type into the source and drain regions of a TFT, and irradiating a linear laser light along a line connecting the source region and the drain region.
In the invention, a semiconductor device including a TFT of a crystal silicon film wherein refractive index dispersion of the crystalline silicon film in a first direction connecting source and drain regions of the TFT is above twice higher than that of the crystal silicon film in a second direction perpendicular to the first direction.
In the invention, a semiconductor device using a crystal silicon film, wherein refractive index dispersion of the crystal silicon film in a carrier moving direction in the semiconductor device is above twice higher than that of the crystal silicon film in a direction perpendicular to the carrier moving direction.
In annealing a semiconductor with a laser light having a linear beam pattern, the characteristics of plural thin films formed in the line direction of the laser pattern can be made uniform by using uniformity of an anneal effect in the line direction of the laser pattern.
The electrical characteristics of the semiconductor device can be improved by coinciding the carrier moving direction of the semiconductor device with the line direction of the line laser pattern. This is because carriers move in an area having uniform crystallinity.
The refractive index dispersion of the crystal silicon film in the carrier moving direction is set to be above twice higher than that in the direction perpendicular to the carrier moving direction, whereby a TFT having high characteristics can be obtained.